Kinetic studies with competitive inhibitors indicate a sequential mechanism for dopamine beta-hydroxylase

Competitive inhibitors have been used to study the kinetic mechanism of action of the multireactant enzyme dopamine beta-hydroxylase (dopamine beta-monooxygenase EC 1.14.17.1). The copper chelator penicillamine, which was found to be a competitive inhibitor of ascorbate, gave noncompetitive inhibition with respect to tyramine. The antidopaminergic drug cis-chlorprothixene, a competitive inhibitor for tyramine, produced uncompetitive inhibition with respect to ascorbate. These findings are consistent with an ordered sequential mechanism, with ascorbate as the initial substrate to add to the enzyme. The pharmacological relevance of dopamine beta-hydroxylase inhibition by penicillamine is also discussed.